Which DHCP/DNS/TFTP Server

Hi,

I need to setup a server to handle DHCP/DNS and TFTP for a network. On
the DHCP side there are a few things that I am not sure of, so I would
appreciate any advice on what DHCP Server to go for. I have looked at
Incognito and it seems to do all I need but it is quite expensive. It
is for a cable network, so there will be at least 2 different scopes in
play - (local ip given to cable modem and public ip given to network
card or router connected to cable modem)

Here is what I need it to be able to do:

* I want to set the lease's to a couple of hours, and set it so that
after the lease has expired they will get a different public IP
regardless of whether the one they are releasing is still available.

* Is it possible to set it up that the modem will always have the same
public ip even if the network card connected to it changes (I know I
can make a reservation for the modem on the local ip scope, but can it
be tied to an address in the public scope?)

I also want to run DNS and TFTP on the server, what would you
recommend? If there was one that could be easily enough managed from a
GUI frontend it would be even better so that I don't need to make every
little change to the system.

Again, any help/advice appreciated,
Regards,
Gerry